Welcome to Audioscrobbler.
Audioscrobbler builds a profile of your musical taste using a plugin for your media player (Winamp, iTunes, XMMS etc..). Plugins send the name of every song you play to the Audioscrobbler server, which updates your musical profile with the new song. Every person with a plugin has their own page on this site which shows their listening statistics. The system automatically matches you to people with a similar music taste, and generates personalised recommendations.
Using this service plus a nice WordPress AudioScrobbler plugin to display the Listening to: links on the right.